This application claims the priority of Korean Patent Application No. 10-2004-0046943, filed on Jun. 23, 2004, in the Korean Intellectual Property Office, which is hereby incorporated by reference for all purposes as if fully set forth herein.
1. Field of the Invention
The present invention relates to an active matrix organic electroluminescent (EL) display device using an organic thin film transistor (TFT), and more particularly, to an active matrix organic EL display device formed in an array structure, including an n-type organic TFT and having an aperture ratio of approximately 100%.
2. Description of the Related Art
FIG. 1 is a plan view of a sub-pixel unit in a conventional active matrix electroluminescent (EL) display device, and FIG. 2 is a cross-sectional view of the sub-pixel unit of the display device taken along line P1 through P7 of FIG. 1.
Referring to the drawings, in conventional silicon thin film transistors (TFTs) 110 and 150 having a semiconductor layer 180 formed of silicon, the semiconductor layer 180 includes a source region and a drain region, both of which are doped by impurities of high concentration, and a channel region formed between these two regions. In addition, the silicon TFTs 110 and 150 include gate electrodes 111 and 151 insulated from the semiconductor layer 180 and positioned to correspond with the channel region, and source electrodes 112 and 152 and drain electrodes 113 and 153 contacting source regions and the drain regions, respectively.
However, the conventional silicon TFTs 110 and 150 are expensive, fragile, and cannot employ a plastic substrate since they are fabricated at a high temperature, for example, 300° C. or higher.
Flat panel display devices such as liquid crystal displays (LCD) or electroluminescent displays (ELD) use TFTs as switching devices and driving devices for controlling and operating pixels. In order to make flat panel display devices large, thin, and flexible, plastic materials are being considered for the substrate instead of glass. However, when plastic is used, manufacturing is difficult because the display device must be fabricated at a temperature below what is necessary for conventional silicon TFTs.
Since an organic TFT solves the above problems, much research is currently being performed into developing organic TFTs including the organic semiconductor layer.
FIG. 3 is a schematic cross-sectional view of an organic EL display device including the conventional organic TFT. Referring to FIG. 3, an organic EL device 210 and an organic TFT 220 are formed on a substrate 200. The organic EL device 210 includes a transparent electrode 211, an organic light emitting layer 212, and a metal electrode 213, which are sequentially formed on the substrate 200, and the organic TFT 220 includes a gate electrode 221 formed on the substrate 200, a dielectric layer 222 formed on the gate electrode 221, an organic semiconductor layer 223 formed on the dielectric layer 222, and a source electrode 224 and a drain electrode 225 located on both sides of the organic semiconductor layer 223 on the dielectric layer 222. The drain electrode 225 is connected to the transparent electrode 211 and the organic light emitting layer 212 of the organic EL device 210.
However, since the organic EL device 210 is horizontal and parallel to the organic TFT 220, the organic EL device 210 has a low apertureratio due to the size of the organic TFT 220. When the aperture ratio is low, the light emitting intensity of the pixels of the display device needs to be increased, which reduces the lifespan of the display device.
To solve the above problem, Korean Patent Publication No. 2003-0017748 discloses an active matrix organic EL display device, in which an organic TFT and an organic EL device are stacked vertically. FIG. 4 is a cross-sectional view of an organic EL display device including the organic TFT described above.
Referring to FIG. 4, an organic EL device 310 and an organic TFT 330 on a substrate 300 are divided vertically by a first insulating layer 320. The organic EL device 310 includes a transparent electrode 311, an organic light emitting layer 312, and a metal electrode 313, sequentially formed on the substrate 300, and the organic TFT 330 includes a gate electrode 331 formed on the first insulating layer 320, a second insulating layer 332 formed on the gate electrode 331, a source electrode 334 and a drain electrode 335 formed on the second insulating layer 332, and an organic semiconductor layer 333 connected to the source and drain electrodes 334 and 335. In addition, the source electrode 334 is connected to the metal electrode 313.
However, the above example is simply an organic EL device having one organic TFT, not an array of a plurality of organic EL devices having a plurality of organic TFTs. Thus it is difficult to use this single organic EL device to realize an actual useable active matrix organic EL display device including a plurality of organic EL devices.
In addition, in the structure disclosed in the Korean Patent Publication No. 2003-0017748, the organic TFT 330 has an inverted coplanar structure, but in order to realize the active matrix organic EL display device using the organic TFT 330 having such structure, complex processes are required, and the display device becomes more complex.